When manufacturing a cage rotor which is used in an induction motor, there has been known a method for integrally forming conductor rods to be disposed in a plurality of slots of a rotor core and end rings connecting each of the conductor rods at both ends in an axial direction by casting such as a die casting and the like. The conductor forming method mentioned above has an excellent productivity and can easily form a conductor having a desired shape, and further the method is suited for improving the characteristic of the motor. For this reason, the method has been widely employed in particular for a compact induction motor. A method for manufacturing a conventional cage rotor of an induction motor will be described with reference to a flow chart shown in FIG. 14 and schematic diagrams shown in FIG. 15(a), FIG. 15(b), FIG. 15(c) and FIG. 15(d).
A cage rotor 12 is provided with a plurality of laminated steel sheets 11 (FIG. 15(a)) which has a plurality of circumferential slots 44, and conductor rods 46 (FIG. 15(d)) extending through the slots 44 of the piled up laminated steel sheets 11.
Then, a method for manufacturing the cage rotor 12 will be described below with reference to the flow chart shown in FIG. 14.
First, a plurality of laminating steel sheet 11 are piled up and stacked so as to form the rotor core 12 by passing a hole formed in a center of the laminated steel sheet 11 through a fixed mandrel 90 (FIG. 15(b)) (Step S11). Next, the thickness of the laminated material is adjusted by measuring the laminated thickness of the stacked rotor core 12 (Step S12). Then, as shown in FIG. 15(b), balance rings 92 are placed on an upper end and a lower end of the rotor core 12 and then those balance rings 92 and the rotor core 12 are tightened each other by fastening portions 94 and 96 (Step S13 and FIG. 15(b))
Next, after pre-heating the rotor core 12 (Step S14), the rotor core 12 is inserted into a metal mold. Then, molten metal such as aluminum and the like is filled in the metal mold at a high speed and a high pressure. That is, a die casting is performed (Step S15). After the molten metal is solidified within the slot 44 of the rotor core 12, the mandrel 90 is pulled out from the rotor core 12 (Step S16).
FIG. 15(d) shows a state that the mandrel 90 is pulled out from the rotor core 12. In general, since the diameter of the mandrel 90 is selected to be smaller by a value d (a degree of play) than the diameter of the hole formed in the center of the laminated steel sheet 11, as shown in FIG. 15(c), the center holes of the plurality of laminated steel sheet 11 stacked to each other are not aligned with each other, as shown in FIG. 15(b) and FIG. 15(d), thereby forming an unevenness on the internal side surface. This unevenness is leveled by later machining, thereby making easier the insertion of the rotor shaft. Further, sometimes, the outer surface of the rotor core 12 may be leveled by removing the unevenness thereon.
However, in the method for casting the conductor of the rotor core mentioned above, there are problems arising from number of kinds of, number of or performance of jigs necessary for casting the metal mold, the mandrel and the like, which affect forming efficiency and quality of the rotor core.
That is, when manufacturing induction motors for various output specifications, there arises a problem such that various kinds of metal molds are necessary. In general, the output of the induction motor is differentiated by changing an axial length while keeping the diameter of the cage rotor constant. Therefore, the rotor cores having the same diameters but having different axial lengths are necessary for many kinds of output specifications. However, for different axial lengths of the rotor core, different metal molds (even if the diameters are the same) are necessary. Accordingly, the different metal molds are necessary for the induction motors having different output specifications, giving rise to the need of various kinds of metal molds.
Further, as to the mandrel too, there arises a problem such that a unique mandrel is necessary for the rotor core having a different length. In casting a rotor core, a mandrel system is generally known as means for fixing the laminated steel sheet piled up to each other. In the mandrel system, since the upper and lower ends of the laminated steel sheet in the axial direction are gripped by the fastening members, and the mandrel for fixing is used as a jig for applying a pressure for fixing, the different fixing mandrels are necessary for the different kinds of rotor core differing in axial length. Further, since the fixing mandrel requires a long time for being removed, it is difficult to use one fixing mandrel in rotation for mass production of the rotor cores, so that the number of fixing mandrels corresponding to the number of the rotor core to be cast will be needed. As a result, there arises a problem that it is necessary to supply a number of fixing mandrels.
Further, in fixing the cast rotor core to the output shaft of the induction motor, there is a problem such that the treatment for the inner periphery of the rotor core is necessary. In the case where laminated steel sheets is stacked on the mandrel so as to form the rotor core, in general, there is a clearance (d in FIG. 15(c)) resulting from a gap between the sleeve diameter of the mandrel and the inner periphery of the laminated steel sheet due to the operability of the stack, so that the bore of the rotor core is hard to be aligned at a time. When mounting the rotor core to the outputted shaft of the induction motor, the rotor core is usually fixed by shrinkage fit. Accordingly, if the rotor core is mounted without aligning the bore of the rotor core by the inner periphery treatment, the output shaft may be curved, or the initial balance of the rotor may be adversely affected.